Dense composite ceramic bodies and method for their production

ABSTRACT

Hard, dense composite ceramic bodies of titanium diboride, boron carbide, silicon carbide and silicon, having a wide variety of utilities, are produced by forming a mixture of titanium diboride, boron carbide and a temporary binder into a desired shape to obtain a coherent green body which is siliconized by heating it in contact with silicon to a temperature above the melting point of silicon, whereupon the molten silicon infiltrates the body and reacts with some of the boron carbide therein to produce silicon carbide in situ.

United States Patent [191 Bailey et al.

[ DENSE COMPOSITE CERAMIC BODIES AND METHOD FOR THEIR PRODUCTION [75]Inventors: Wallace 0. Bailey, Westland, Mich;

Carl H. McMurtry, Lewiston; Bruno R. Miccioli, North Tonawanda, both ofNY.

[73] Assignee: The Carborundum Company,

Niagara Falls, NY.

22 Filed: Sept.15, 1972 21 App]. No: 289,378

Related US. Application Data [62] Division of Ser. No. 135,392, April19, 1971.

[52] US. Cl 264/29, 75/203, 106/44, 106/55,'252/516, 264/60 [51] Int.Cl. C04b 35/56, C04b 35/58 [58] Field of Search 264/29, 60; 106/43, 44,106/55, 56; 252/516, 520; 51/307 75/200, 203

[5 6] References Cited UNITED STATES PATENTS 2.546.142 3/1951 Watson...106/55 1 Jan. 7, 1975 Schildhaver et al. 106/44 2,938,807 5/1960Anderson 106/44 3,153,636 10/1964 Shanta et al. 106/43 3,325,300 6/1967Wise et al. 106/44 3,459,566 8/1969 Wilson et a1 264/29 PrimaryExaminer-Jeffery R. Thurlow Attorney, Agent, or Firm-David- E.Dougherty; Herbert W. Mylius ABSTRACT 13 Claims, No Drawings SUMMARY OFTHE INVENTION The present invention relates to hard, dense (i.e.,substantially nonporous) composite ceramic materials consistingessentially of titanium diboride, boron car bide, silicon carbide andsilicon, and to a method for the production of such composite ceramicmaterials. In accordance with the method of the invention, hard, densebodies of such composite ceramic materials may be readily produced inany of a wide variety of shapes without resorting to hot pressing, whichis disadvantageous in being relatively expensive and in being limited tothe production of relatively simple shapes.

Briefly, the method-of the invention comprises preparing a substantially homogeneous initial mixture of granular titanium diboride andgranular boron carbide in a proportion of from about :95 to about 95:5and a temporary binder; forming the initial mixture into a desired shapeby pressing, extruding, investment or slip casting or any other suitablemethod; wetting the temporary binder, if necessary, to impart sufficientcoherence to the shaped green body to permit further processing; andsiliconizing thecoherent green body by heating it in contact withsilicon to a siliconizing temperature above the melting point ofsilicon.Thereupon, the silicon in the molten state, infiltrates the body andundergoes a rather complex reaction with some of the boron carbide,producing some silicon carbide in situ. The titaniumdiboride apparentlydoes not undergo any reaction. To the extent that interstices'exist inthe body between the titanium diboride, the remaining boron carbide andthe newly-formed silicon carbide, the interstitial space is permeated byfree silicon. The silicon must be at least about 5:95 so that sufficienttitanium diboride is present in the final body to impart its desirableproperties thereto; and the proportion must not exceed about 95:5 sothat sufficient boron carbide is present in the green body to react withthe silicon and produce silicon carbide.

In addition to being hard and dense, the composite ceramic bodies of theinvention possess many other desirable properties, being refractory,tough, wearresistant, abrasion-resistant, and resistant to most acidsand alkalis. The oxidation resistance of the bodies tends to increasewith increasing titanium diboride content, bodies of high titaniumdiboride content having particularly outstanding oxidation resistance.These desirable properties render the bodies of the invention, insuitable shapes, useful in a wide variety of wearresistant and otherapplications, including, for example, extrusion dies, sandblast nozzles,cutting'tool tips, abrasives, suction box covers for paper-makingmachines and the like. The bodies are characterized by a carbide andfree silicon bond the other materials and an extremely hard anddense-body is formed.

The temporary binder employed may be such as to be completely dissipatedduring the siliconizing heating cycle; or it may be a carbonizablematerial which will leave a carbon residue in the body upon heating, inwhich case the silicon will also react with substantially all of theresidual carbon to produce silicon carbide, and accordingly theresulting body will generally have a somewhat higher silicon carbidecontent and a somewhat lower free silicon content. The same result maybe obtained by incorporating in the initial mixture a small amount,i.e., up to about 10 percent of the combined weight of the titaniumdiboride and boron carbide, of finely divided carbon of any suitablevariety such as powdered graphite. If desired, both finely dividedcarbon and a carbonizable binder may be included in the initial mixture.

The hard dense composite ceramic bodies produced in accordance with theforegoing method consist essentially of from about 2 to about 80 percenttitanium diboride, from about 2 to about 70 percent boron carbide, fromabout 5 to about 30 percent silicon carbide,

, and from about 3 to about 20 percent free silicon, the

precise composition of a given body depending primarily upon thecomposition of the initial mixture. Such composite bodies have aspecific gravity within the range from about 2.6 to about 4.1,increasing with increasing titanium diboride content. The proportion oftitanium diboride to boron carbide in the initial mixture high Youngsmodulus of elasticity ranging from about 3 X 10 kg./sq. cm. to about 4 X10 kg./sq. cm. which, together with their other desirableproperties,'renders the bodies, in suitable shapes, highly useful aspersonnel, vehicular and aircraftarmor. It has also been found that thebodies of the invention, especially those having a relatively hightitanium diboride content, are quite electrically conductive andextremely resistant to corrosion by molten aluminum and aluminum alloys,thus they find utility as current conducting elements foruse in contactwith molten aluminum and alloys thereof, such aselectrodes for refiningaluminum. They also find utility as various parts of pumps used forpumping molten aluminum and alloys thereof, such as pistons,cylinders,impellers, bearings,and the like.

Titanium diboride itself is known to be electrically conductive,resistant to moltenaluminum,and to possess desirable properties as aceramic'armor material.

Accordingly, in order to avail of these properties when the bodies ofthe invention are to be used in armor or aluminum applications, it ispreferred that the proportion of titanium diboride to boron carbide inthe initial mixture be at least about 65:35; and it is also preferred,

especially for armor, that this proportion not exceed about 85: l 5, sothat sufficient boron carbide is present to result in the production ofenough silicon carbide tocontribute to a particularly strong bondingphase. The preferred composite bodies of the-invention are produced frominitial mixtures containing titanium diboride and boron carbide in thispreferred range of porportions, and consist essentially of from about 45to about percent titanium diboride, from about 10 to DESCRIPTION OFPREFERRED EMBODIMENTS The invention will now be described in greaterdetail partly with reference to the following examples, which areintended to illustrate, and not to limit the scope of,

the invention.

EXAMPLE 1 A mixture is prepared consisting of 8,500 g. of granulartitanium diboride having a particle size of 45 microns and finer and1,500 g. of granular boron carbide comprising 45 percent with 'a medianparticle size of lmicrons and ranging from 70 microns to 140 microns, 23percent with a median particle size of 65 microns ranging from 40microns to 100 microns, and 32 percent with a particle size of micronsand finer. Thereto is added 1,100 g. of a temporary carbonizablebinderconsisting of 600 g. of a liquid thermosetting phenol-formaldehyderesin of the kind typified by that Chemicals, Inc. under the trade nameVarcum B-178 and 500 g. of furfural, the latter also serving to addplasticity ,to the mix. The mixture is blended until it is sold byvarcum Chemical -Division of 'Reichhold 1 substantially homogeneousandthe resulting mix, or in- The green body is placed on a graphitesupporting plate in the chamber of an induction heated furnace, and1,530 g. of granular silicon'is distributed evenly over the surface ofthe green body. The furnace chamber is evacuated to a pressure of about50 microns of mercury, and the power source to the induction coils ofthe furnace is turned on. The reduced pressure is maintained throughoutthe heating cycle. An optical pyrometer sighted on the piece is used toascertain the temperature of the piece as the temperature rises. Thetemperature of the piece is brought to 1,470C; the silicon melts, andthe molten silicon infiltrates the piece quite abruptly and reacts withsome of the boron carbide and with substantially all of the residualcarbon from the binder to produce silicon carbide. The power isimmediately turned off and the furnace and its con tents are permittedto cool to room temperature.

The ceramic plate thus produced has a specific gravity of 3.5, anelectrical resistivity of 5 X 10" ohm-cm., a flexural strength of 1,400kg./sq. cm., a Young's modulus of elasticity of 3.4 X 10 kg./sq. cm.,and a shear modulus of 1.7 X 10v kg./sq. cm. Elemental analysis fortotal titanium, total boron, and free silicon indicates that the piececonsists essentially of 66 percent titanium diboride, 12 percent boroncarbide, 11 percent silicon carbide, and 11 percent free silicon. X-raydiffraction analysis using monochromatic copper K-alpharadiation'indicates that boron carbide present is of two types, thefirst type having a diffraction pattern corresponding to normal 8 C, andthe second type having a diffraction pattern of boron carbide with anexpanded lattice, this second type apparently being a boron carbide typesolid solution. The x-ray diffraction analysis also indicates thepresence oftitanium diboride, silicon carbide, and free silicon. The'material exhibits excellent oxidation resistance, a portion of theplate showing substantially no weight change upon exposure to air atl,00.0C for 125 hours. No effect is observed upon immersion of a portionof the plate in molten aluminum EXAMPLE 2 A mixture is preparedconsisting of 55.5 kg. of granular titanium 'diboride havin'g a'particle size of 45 'microns and less and 23.8 kg. of granular boron carbide 4having a particle size of 10 microns. and less. Theretois added 5.6 kg.ofa noncarbonizable temporary binder consisting of a polyethylene glycolhaving an average molecular weight of 6,000 to 7,500 of the kindtypified by that soldby Union Carbide Corp. under the trade nameCarbowax 6000. Approximately 4 l. of methanol is added to the mixture todissolve the binder and aid the dispersion of the granular materialstherein. The

mixture is blended until it is substantially homogeneous and themethanol is allowed to evaporate. The mix is then passed through acoarse screen to break up any agglomerates.

A 2,850 g. quantity of the screened mix is placed in a 9 inch square(22.8 cm. square) mold and pressed at 6,000 psi. (420 kg./cm. sq.) toform a-coherent-green body 0.895inch (2.27 cm.) thick'having a specificgravity of 2.4. The body is then siliconized as-as described in Example1, employing 2,420 g. of granular silicon and a temperature of 1,500C.The resulting plate has a specific gravity of 3.2, a Youngs modulus ofelasticity of 3.5 X 10 kg./sq. cm.,,a flexural strength of 1400 kg./sq.cm., and an electrical resistivity of l X 10 ohm-cm. The piece issubstantially the same as that produced in Example 1 with respecttoarmor capability, oxidation resistance, and'resistance to aluminum andaluminum alloys. Elemental analysis indicates that the piece consistsessentially of 50.7 percent titanium diboride, 21.8 percent boroncarbide, 13.5 silicon carbide and 14 percent free silicon.

EXAMPLES A mixture is prepared consisting of 10 g. of granular titaniumdiboride having a particle size of 45 microns and less andg. of granularboron carbide having the same range of particle sizes as that employedin Example l. Thereto is added 1 l g. of the same carbonizable temporarybinder as employed-in Exam le 1 and the materials are mixed until asubstantially omogeneous initial mixture is formed. The mix is passedthrough a coarse screen to break up any agglomerates.

A 10 g. portion of the screened mix-is placed in a steel mold 3 inches(7.6 cm.) long and 0.5 inch (1.27 cm.) wide and compressed at about 4000psi. (280 kg./sq. cm.) into a bar 025 i ch (0.64 m.) thick. The binderis set by heating the piece for 16 ours at C and for 16 hours at C. Theresulting coherent X kg./sq. cm., which is comparable to that of thepieces produced in Examples 1 and 2, indicating that it would be equallysuitable for use as armor. Elemental analysis indicates that the piececonsists essentially of 6.3 percent titanium diboride, 56.7 percentboron carbide, 24 percent silicon carbide and 13 percent free sillCOI'l.

EXAMPLE 4 A mixture is prepared consisting of 90 g. of granular titaniumdiboride having a particle size of 45 microns and less and 10 g. ofgranular boron carbide having the same range of particle sizes as thatemployed in Example 1. Thereto is added 11 g. of the same carbonizabletemporary binder as employed in Example 1, and the materials are mixeduntil substantially homogeneous. The mix is passed through a coarsescreen to break up any agglomerates.

A 16 g. portion of the initial mixture is placed in a mold having thesame dimensions as that employed in Example 3 and compressed at 4,000psi. (280 kg./sq. cm.) to a bar 0.25 inch (0.64 cm.) thick. The binderis cured as in Example 3, the resulting coherent green body having aspecific gravity of 2.80. The piece is siliconized as in Example 3,employing 13.6 g. of granular silicon. The resulting ceramic tile has aspecific gravity of 3.54, and a Youngs modulus of elasticity of 3.5 X 10kg./sq. cm. which indicates its suitability for use as armor. Elementalanalysis indicates that the piece consists essentially of 71.1 percenttitanium diboride, 7.9 percent boron carbide, 8 percent silicon carbideand 13 percent free silicon. I

Dense bodies may be prepared according to the invention from an initialmixture containing titanium diboride and boron carbide grain of asingle, uniform particle size. However, green bodies formed from suchmixtures have considerably more interstitial space unoccupied bytitanium diboride and boron carbide than bodies formed from mixturescontaining particles of varying sizes, and therefore tend to have acomparatively high free silicon content after siliconization. While thisis often unobjectionable, for certain applications it may be desirableto reduce or minimize the free silicon content. As already noted, auseful expedient is to employ a carbonizable binder-and/or carbon in theinitial mixture. Whether or not this is. done, however,

it is usually preferred to employ titanium diboride of varying grainsizes, or boron carbide of varying grain sizes, and preferably both asin the examples, the variety being such as to permit dense packing inaccordance with known principles, thus tending to maximize the specificgravity of the green body and the siliconized body and to reduce thevolume of interstitial space available for occupancy by free silicon.Composite bodies containing as little asabout 3 percent free silicon canbe produced by suitably selecting the grain sizes in addition toproviding carbon in the green body.

In general, the finer the boron carbide, themore reactive it is.Accordingly, it is often desirable to employ at least some boron carbidewhich is very fine, i.e., l0 micronsand finer, and there is seldom anyadvantage to using boron carbide having a particle size greater thanabout 150 microns. The titanium diboride does not appear to undergo anyreaction, and there is no critical upper limit of particle size; butwhen certain shaping techniques such as extrusion are to be employed, itis often'desirable to employ relatively fine titanium diboride having aparticle size of about 125 microns or less, the same being true of theboron carbide in such cases.

The temporary binder may be selected from among a wide varietyof-materials recognized as suitable for such use, for example,polyethylene glycols and methoxypolyethylene glycols such as those soldby Union Carbide Corp. under the trademark Carbowax, polyvinyl alcohol,and where a carbonizable binder is desired, phenolformaldehyde and otherphenolic resins, epoxy resins, dextrin, starches-and the like.

The binder is employed in an amount sufficient to give an initialmixture which is of the proper consistency for forming into the desiredshape by the method to be employed. Even if the binder is of thecarbonizable type, this amount is usually established without regard tothe amount of carbon present therein, since finely divided'carbon can beincorporated in the initial mixture in an amount sufficient to provide.the total quantity of carbon desired in the green body at the time ofsiliconization. However, it is usually preferred that the carbonizablebinder have a high carbon content so that as much of the carbon aspossible comes from this source, such carbon generally being more'finelydivided and dispersed than the carbon added to the initial After theingredients of the initial mixture have been blended together to form asubstantially homogeneous mixture which, if necessary, is passed througha coarse screen to break up any agglomerates, the mixture is formed intothedesired shape and the binder is set, if necessary, to produce acoherent green body.

A particular advantage of the method of the invention lies in the factthat any of a wide variety of methods may be employed to shape the mix,and any of a wide variety of simple or complex shapes may thereby beobtained. The method of choice will depend primarily upon the shapedesired. The composition of the initial mixture may be varied,especially in respect of the binder and the particle sizes of thetitanium diboride and boron carbide, to obtain the most suitable mix forthe particular method of forming to be employed. As illustrated in the'examples, relatively simple shapes such as flat tiles may readily beproduced by compression molding, preferably using sufficient pressure toobtain substantially maximum specific gravity in the green body toreduce the interstitial space to a minimum. Impact molding and rammingmay be employed to form complex solid shapes,-slip casting andinvestment casting also being useful to form complex shapes such asarmor helmets, armor leg sections, armor vest sections, etc. Tubes, rodsand the like may be readily formed by extrusion. Extrusion may-also beemployed to produce a fine strand of the initial mixture, which may becut into short lengths to produce grain which may be siliconized toobtain abrasive grain.

When the initial mixture has been formed into the desired shape, thebinder is set, if necessary, to obtain a coherent green body, underconditions suited to the particular binder material employed. Binderssuch as polyvinyl alcohol, polyethylene glycols and methox ypolyethyleneglycols require I no setting, although evaporation of the solvent may benecessary if one is employed. To set binders such as phenol-formaldehyderesins, curing of the resin is usually carriedout by employing a heatingcycle such as illustrated in the examples, the temperature beingincreased gradually or stepwise to permit gradual dissipation of thevolatiles that are produced during curing without cracks being formed inthe body. The coherent green body should have at least adequate strengthto permit further handling and processing, and preferably should bestrong enough to permit machining, if desired.

Instead of, or in-addition to, incorporating carbon and/or acarbonizable binder in the initial mixture, carbon may be incorporatedin the coherent green body by impregnating it with a carbonizablematerial such as a phenolic resin, which, upon subsequent-heating,produces carbon, which reacts with the silicon during siliconizationthereby'reducing the proportion of free silicon in the final body.

When a binder of the noncarbonizable type is employed, it is merelydissipated by the heat during the siliconization step. When acarbonizable binder is employed, the binder in the coherent green bodymay be carbonized by heating the body to a sufficiently high temperatureto'effect carbonization. Since volatiles are dissipated from the bodyduring carbonization, it may be desirable to control the carbonizingheating cycle by providing for a slow rate of temperature increase topermitthe escape of the volatiles without cracking the body. The needfor such control tends to increase with increasing size and thickness ofthe body. As may be seem from the examples, no specific temperaturecontrol is required under the conditions there set forth for bodiesof-the described dimensions; and while carbonization may, if desired, becarried out as a separate step, it is usually most convenientlyaccomplished during the siliconizing heating cycle.

Siliconization of the green body is effected by heating .it in contactwith silicon to a temperature above the melting point of silicon,whereupon the silicon in the molten state, infiltrates the body andreacts with some of the boron carbide therein and with substantially allof the carbon present, if any, producing silicon carbide in situ,the'reactions being very rapid. If desired, the green body may simply beimmersed in molten silicon for a brief time until the body is heated tothe required temperature, whereupon infiltration and reaction occur, butthis technique usually results in a siliconized body coated withsilicon, which is extremely difficult to remove. Preferably, therefore,the siliconization is carried out as described in the examples, byplacing the greenbody in a suitable furnace with the appropriate amountof granular silicon spread upon the top of the body, or alternatively,by placing the green body on a bed of the appropriate amount of siliconin a crucible.

Preferably, the amount of silicon employed is carefully controlled. Whentoo little silicon is used, the core of the body remains unsiliconized,although this may not be important for some uses of the bodies, such ascertain wear-resistant applications in which only the surface propertiesof the bodies are of significance.

Conversely, when too much silicon is employed, the excess tends to buildup on the outside of the piece as a coating which is extremelydifficult, although possible, to remove. The precise amount of siliconto be used for a given piece cannot be computed with exactitude, sinceit is impossible to precisely predict the extent of the reaction betweenthe silicon and the boron carbide and the amount of silicon carbide thatwill be produced thereby. It is also difficult or impossible to predicthow much free silicon will be present in the siliconized body. However,a reasonably close approximation of the requisite amount of silicon canbe made by subtracting the measured specific gravity of the coherentgreen body from the approximate desired specific gravity of thesiliconized body and multiplying 'by the volume of the body, thuscomputing the weight of silicon needed to give the desired weightincrease, assuming as is true that there is no appreciable change in thedimensions of the green body upon siliconization. The precise optimumamount of silicon is best determined experimentally for a green body ofgiven dimensions specific gravity and composition, using the calculatedapproximation as a starting point which is subject to modification andfurther experimental refinement.

Preferably, the siliconization is carried out in a nonoxidizingatmosphere such as argon, helium, neon or the like, and still morepreferably in a vacuum, in order to avoid oxidation of the titaniumdiboride and boron carbide in the body. Vacuum is especially preferred,

since it aids in the removal of any air trapped within the piece andthereby hastens infiltration of the silicon..

Therefore, the higher the vacuum, the better, pressures below about 1mm. of mercury being preferred, and pressures below. about microns ofmercury being still more preferred. 1'

While siliconizationtemperatures greatly in excess of the melting pointthough below the boiling point, ofsilicon may be used, there isgenerally no advantage in doing so, and it is preferred to use thelowest possible temperature in order to minimize the time required. Whenthe body has attained the desired temperature, it may be held there fora period of time, if desired, but such holding is unnecessary.

The composition and structure of the composite bodies of the inventionappear to be quite complex. Elemental analysis and x-ray diffractionanalysis of the bodies produced in Examples 1-4 and of other bodiesproduced in accordance with the invention indicate that the bodiesinvariably consist essentially of titanium diboride, boron carbide,silicon carbide and free sili con. X-ray diffraction indicates that theboron carbide is almost always of two types, both types having a boroncarbide type rhombohedral structure, but one having a diffractionpattern corresponding to .normal 8 C, and the other having a diffractionpattern of boron carbide with an expanded lattice and being of lessdeterminate composition but containing at least boron and carbon andpossibly some silicon. The latter type isusually present in but smallamounts when the siliconization is carried out at a temperature onlyslightly above the melting point of silicon and without holding thepiece at the siliconization temperature, tending'to increase in amountwith increasing siliconization temperature and holding time, and beingpresent to the virtual exclusion of the normal type under extremesiliconization conditions. It is also generally observed that thepercentage of free silicon present in the final body decreases withincreasing temperature and holding time during siliconization, acorresponding increase in silicon carbide content also being observed.In addition to being dependent upon the precise composition of theinitial mixture, the composition of the siliconized bodies also dependsupon the degree of porosity present in the green body. Relatively highporosity green bodies tend to result in siliconized bodies having arelatively high concentration of free silicon, the titanium diboride andboron carbide concentrations being somewhat reduced and the siliconcarbide concentration likewise being slightly reduced in general. Theconverse effect is observed with relatively low porosity green bodies.Minor impurities such as iron and calcium which may be present in thegranular silicon, boron carbide and/or titanium diboride employed mayalso be present in the composite bodies.

Percentages and proportions referred to herein are percentages andproportions by weight, except as otherwise expressly stated or clearlyindicated by the content.

While the invention has been described herein with reference to certainexamples and preferred embodiments, it is to be understood that variouschanges and modifications may be made by those skilled in the artwithout departing from the concept of the invention, the scope of whichis to be determined by reference to the following claims.

We claim:

1. A method of making a dense, composite ceramic body consistingessentially of from about 2 to about 80 percent titanium diboride, fromabout 2 to about 70 percent boride carbide, from about to about 30percent silicon carbide, and from about3 to about 20 percent freesilicon, said method comprising preparing a substantially homogenousinitial mixture of granular titanium diboride and granular boron carbidein a proportion of from about 5:95 to about 95:5 and a temporary binder;forming said mixture into a desired shape; setting the binder, ifnecessary, to obtain a coherent green body; and siliconizing said greenbody by heating it in contact with silicon to a siliconizing temperatureabove the melting point of silicon whereby the silicon, in the moltenstate, infiltrates the body and reacts with some of the boron carbidetherein to produce silicon carbide in situ, and said free siliconoccupies interstices between said titanium diboride, boron carbide, andsilicon carbide.

2. A method as set forth in claim 1 wherein said granular titaniumdiboride consists'of a variety of particle sizes.

3. A method as set forth in claim 1 wherein said granular boron carbideconsists of a variety of particle sizes.

4. A method as set forth in claim 1 wherein both said granular titaniumdiboride and said granular boron carbide consists of a variety ofparticle sizes.

5. A method as set forth in claim 4 wherein said siliconizing is carriedout in a nonoxidizing atmosphere. 6. A method as set forth in claim 4wherein said siliconizing is carried out in a vacuum at-a pressure ofless than about 1 mm. of mercury with a controlled amount of silicon.

7.A method as set forth in claim 6 wherein said temporary binder iscarbonizable, and said binder is carbonized to produce carbon in saidgreen body prior to siliconizing it, and said silicon reacts withsubstantially all of said carbon to produce silicon carbide.

8. A method as set forth in claim 6 wherein finely divided carbon isincorporated in said initial mixture, and said silicon reacts withsubstantially all of said carbon to produce silicon carbide.

9. A method as set forth in claim 6 wherein said coherent green body isimpregnated with a carbonizable material and said material is carbonizedto produce carbon in said green body prior to siliconizing it, and saidsilicon reacts with substantially all of said carbon to produce siliconcarbide.

10. A method as set forth in claim 7 wherein said coherent green body isimpregnated with a carbonizable material and said material is carbonizedto produce carbon in said green body prior to siliconizing it, and saidsilicon reacts with substantially all of said carbon to produce siliconcarbide. i

11. A method as set forth in claim 6 wherein said granular titaniumdiboride and said granular boron carbide are present in the initialmixture in a proportion of from about 65:35 to about :15.

12. A method as set forth in claim 7 wherein said granular titaniumdiboride and said granular boron carbide are present in theinitialmixture in a proportion of from about 65:35 to about 85:15.

13. A method as set forth in claim 9 wherein said granular titaniumdiboride and said granular boron'carbide are present in the initialmixture in a proportion of from about 65:35 to about 85:15.

1. A METHOD OF MAKING A DENSE, COMPOSITE CERAMIC BODY CONSISTINGESSENTIALLY OF FROM ABOUT 2 TO ABOUT 80 PERCENT TITANIUM DIBORIDE, FROMABOUT 2 TO ABOUT 70 PERCENT BORIDE CARBIDE, FROM ABOUT 5 TO ABOUT 30PERCENT FREE SILICON, CARBIDE, FROM ABOUT 3 TO ABOUT 20 PERCENT FREESILICON, SAID METHOD COMPRISING PREPARING A SUBSTANTIALLY HOMOGENOUSINITIAL MIXTURE OF GRANULAR TITANIUM DIBORIDE AND GRANULAR BOROM CARBIDEIN A PROPORTION OF FROM ABOUT 5:95 TO ABOUT 95:5 AND A TEMPORARY BINDER;FORMING SAID MIXTURE INTO A DESIRED SHAPE; SETTING THE BINDER, IFNECESSARY, TO OBTAIN A COHERENT GREEN BODY; AND SILICONIZING SAID GREENBODY BY HEATING IT IN CONTACT WITH SILICON TO A SILICONIZING TEMPERATUREABOVE THE MELTING POINT OF SILICON WHEREBY THE SILICON, IN THE MOLTENSTATE, INFILTRATES THE BODY AND REACTS WITH SOME OF THE BOROM CARBIDETHEREIN TO PRODUCE SILICON CARBIDE IN SITU, AND SAID FREE SILICONOCCUPIES INTERSTICES BETWEEN SAID TITANIUM DIBORIDE, BOROM CARBIDE, ANDSILICON CARBIDE.
 2. A method as set forth in claim 1 wherein saidgranular titanium diboride consists of a variety of particle sizes.
 3. Amethod as set forth in claim 1 wherein said granular boron carbideconsists of a variety of particle sizes.
 4. A method as set forth inclaim 1 wherein both said granular titanium diboride and said granularboron carbide consists of a variety of particle sizes.
 5. A method asset forth in claim 4 wherein said siliconizing is carried out in anonoxidizing atmosphere.
 6. A method as set forth in claim 4 whereinsaid siliconizing is carried out in a vacuum at a pressure of less thanabout 1 mm. of mercury with a controlled amount of silicon.
 7. A methodas set forth in claim 6 wherein said temporary binder is carbonizable,and said binder is carbonized to produce carbon in said green body priorto siliconizing it, and said silicon reacts with substantially all ofsaid carbon to produce silicon carbide.
 8. A method as set forth inclaim 6 wherein finely divided carbon is incorporated in said initialmixture, and said silicon reacts with substantially all of said carbonto produce silicon carbide.
 9. A method as set forth in claim 6 whereinsaid coherent green body is impregnated with a carbonizable material andsaid material is carbonized to produce carbon in said green body priorto siliconizing it, and said silicon reacts with substantially all ofsaid carbon to produce silicon carbide.
 10. A method as set forth inclaim 7 wherein said coherent green body is impregnated with acarbonizable material and said material is carbonized to produce carbonin said green body prior to siliconizing it, and said silicon reactswith substantially all of said carbon to produce silicon carbide.
 11. Amethod as set forth in claim 6 wherein said granular titanium diborideand said granular boron carbide are present in the initial mixture in aproportion of from about 65:35 to about 85:15.
 12. A method as set forthin claim 7 wherein said granular titanium diboride and said granularboron carbide are present in the initial mixture in a proportion of fromabout 65:35 to about 85:15.
 13. A method as set forth in claim 9 whereinsaid granular titanium diboride and said granular boron carbide arepresent in the initial mixture in a proportion of from about 65:35 toabout 85:15.